So why is the 66-year-old, who has never held a ministerial office and spent much of his 30-plus-year parliamentary career as a campaigner, making Blair and Blairites nervous? Here is the Islington North MP views in his own words.

“I think as a party we have to challenge the economic orthodoxy of cuts and the poverty that goes with it and say to the people of this country we want an expanding economy, we want a housing agenda which ensures that everyone actually gets somewhere decent to live.”

He was one of 48 Labour MPs - and the only Labour leadership candidate - to defy the Labour whip and vote against welfare cuts outlined in the Budget by George Osborne.

“In the absence of rent control all that’s happening in central London is that families who access benefits to pay their rent cannot get enough money to pay their rent. They’re forced to move away and it’s leading to a social cleansing of much of central London. If we can’t control rents then the very least we can do is keep families together.”

Syria: opposes bombing

A staunch opponent of the Iraq War, he has made clear resistance to British involvement in air strikes in Syria after it emerged British pilots have been embedded with US and Canadian forces taking part in raids.

“It absolutely has no authority for British forces to be involved in Syria. The excuse that these individual pilots are embedded with other airforces seems to me a very difficult and dodgy argument

“I would have thought the Prime Minister comes to Parliament, makes a clear statement on it, and then Parliament can then make a decision because this has the danger of developing into British direct military involvement by air forces and then by ground forces.”

"To those who say that it is all for our security and that our security is enhanced by nuclear weapons, let me say this. If we follow that argument, any country in the world can say, 'We need nuclear weapons'. Iceland could say it wants them; Paraguay could say it wants them; Japan could say it must have nuclear weapons - the list goes on, the countries get bigger and the possibilities become more dangerous."

Ireland: wants to reunite Ireland

He supported ending British status for Northern Ireland, and was heavily criticised for inviting Gerry Adams and other Sinn Féin members to the House of Commons shortly after the Brighton bombing in 1984. He defended the gesture in recent weeks.

"They were former prisoners who had come out of prison, women who had come out of prison, came to parliament actually at a meeting that had been arranged long before to talk about prison conditions and rehabilitation of prisoners."

Has also defended welcoming his "friends" from terrorist groups Hezbollah and Hamas into the corridors of British power.

“I want to apologise on behalf of the Labour Party to the last generation of students for the imposition of fees, top-up fees and the replacement of grants with loans by previous Labour governments. I opposed those changes at the time – as did many others – and now we have an opportunity to change course.”

Re-nationalisation: railways and public utilities brought back into public ownership

"In crude terms since privatisation the public have paid £10 billions in subsidies and the operators have posted aggregate profits of £1 billion. Enormous amounts are being invested in new track and signalling and will be for at least the next decade. No sensible person would oppose this but they should question why the beneficiaries of this have to be private companies running in a protected environment."